Cochlear implants restore some hearing capabilities to individuals with sensorineural hearing loss by directly electrically stimulating remaining auditory neurons. However, a significant portion of the electrical current spreads within the cochlea, decreasing the spatial selectivity of stimulation and, thus, limiting patient performance. An alternative method of stimulation, using optical energy, has been shown in the gerbil cochlea in vivo. Optical stimulation is a non-contact method capable of achieving much more spatially selective neural stimulation, which could improve cochlear implant patient performance. To achieve the goal of building an optical cochlear implant, certain objectives will be addressed: (1) demonstrate an improved spatial resolution of optical stimulation as compared to electrical stimulation and (2) demonstrate optical stimulation at high repetition rates. Aim (1) will utilize a tone-on-light masking technique and immunohistochemical analysis of the transcription factor, c-fos, to demonstrate the size of the neural populations stimulated. Aim (2) will utilize the variable-wavelength, variable-repetition rate Free Electron Laser to demonstrate optical stimulation of auditory neurons at high repetition rates. [unreadable] [unreadable] [unreadable]